This will make it possible to cut away additional
levels of directories when applying patches from repos
that have a different directory hierarchy than Chromium.

BUG=438952
TESTED=Executed successfully in a local checkout using
(cwd was the dir above src/):
~/depot_tools/apply_issue.py --root_dir src/third_party/webrtc --issue 1978193002 --server https://codereview.webrtc.org --force --ignore_deps -v -v --no-auth --patchset 20001 --extra_patchlevel=1

Review-Url: https://codereview.chromium.org/1988583002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@300615 0039d316-1c4b-4281-b951-d872f2087c98